Vinyl chloride resin can be used for various uses since the hardness thereof can be easily adjusted by adding plasticizers. In particular, since hard vinyl chloride resin compositions having no plasticizer, or semi-hard vinyl chloride resin compositions having only a small amount of plasticizer have excellent rigidity, they are widely used for building materials or the like. Therefore, in the case of hard vinyl chloride resin compositions having no plasticizer and semi-hard vinyl chloride resin compositions having only a small amount of plasticizer, more advanced performances are required not only in working at high temperatures and pressures but also in heat stability and weather resistance as molded articles.
In addition, hard transparent molded articles obtained from hard vinyl chloride resin compositions have been used for a wide range of products such as containers, industrial boards, decorative boards, film and sheets, wherein glass-like transparencies are required. On the contrary, it has been known that vinyl chloride resin has disadvantages in that it is insufficient in stability against light and heat, therefore, a decomposition mainly caused by a dehydrohalogenation easily occurs when heat molding is carried out or when the products are used. In order to improve these disadvantages, attempts have been conventionally made wherein various stabilizers such as an organic acid metallic salt, an organic tin compound, an organic phosphite compound, an epoxy compound, a β-diketone compound, an antioxidant and an ultraviolet absorber are blended to improve the stability of vinyl chloride resin.
Conventionally, heavy metal-based stabilizers such as lead-based and cadmium-based stabilizers have been used from a viewpoint of an advantage in terms of cost or the like. Recently the interest in environmental problems has been growing, therefore, the toxicity and affection of the heavy metal or the like on the environment have come to problems and there has been a tendency to use barium-zinc composite stabilizers. However, in recent years an affection of barium on the environment have started to cause concern. Therefore, the replacement to further lower-toxic composite stabilizers such as calcium-zinc, magnesium-zinc or calcium-magnesium-zinc composite stabilizers is required.
From the viewpoints of environmental problems, a method of adding an organic acid calcium salt, an organic acid zinc salt, a basic phosphorous acid salt and/or a hydrotalcite compound into the vinyl chloride resin (Patent document 1), and a method of adding an organic acid alkaline earth metallic salt, an organic acid zinc salt, a basic zinc phosphate and/or a hydrotalcite compound into the vinyl chloride resin (Patent document 2) were proposed. However, even though these compounds are used in combination, the vinyl chloride resin, which can satisfy the performances of heat resistance and the like, could not be obtained. In addition, in these patent documents, there were no description with regard to the transparency at all, and also there was no suggestion of improvement in transparency at all.
Furthermore, there are proposed a vinyl chloride resin composition, which is able to foam, for foaming molded products wherein an inorganic filler, a zinc compound, a zinc-modified hydrotalcite compound and a thermal decomposition type organic foaming agent are added (Patent document 3), and a polyvinyl chloride resin composition for food wrapping wherein an adipic acid ester plasticizer, a calcium salt of organic carboxylic acid and a zinc-modified hydrotalcite compound are blended (Patent document 4). However, since there are no descriptions with regard to transparency at all in any of these documents, any knowledge with regard to the improvement of transparency could not be obtained from these documents.
In addition, tin-based stabilizers are proposed as a stabilizer which can enhance the transparency (Patent documents 5 and 6). However, tin is also not preferable from the viewpoints of affection on the environment and toxicity. Moreover, a lot of tin-based stabilizers are liquid. Therefore, when these tin-based stabilizers are used, there were problems in that not only lubricating properties of molten resin during mold-working decrease, but also the heat distortion temperature of molded object (a softening point) drops.
In cases of using stabilizers such as calcium-zinc-based, magnesium-zinc-based or calcium-magnesium-zinc-based stabilizers, coloring properties often become a problem.
As seen above, a vinyl chloride resin composition, which does not affect the environment negatively, and can provide a molded article excellent in coloring resistant properties and transparency has not been known yet.